Semiconductor power device

ABSTRACT

A semiconductor power device, comprising: a substrate; a first semiconductor layer with a first lattice constant formed on the substrate; a first grading layer formed on the first semiconductor layer and comprising a first portion; a second grading layer formed on the first grading layer; a second semiconductor layer with a second lattice constant formed on the second grading layer; a first interlayer formed in the first grading layer and adjacent to the first portion of the first grading layer; and a second interlayer formed in the second grading layer; wherein the first interlayer comprises a first superlattice including a series of Al x1 Ga 1-x1 N/Al y1 Ga 1-y1 N alternate layers, (x1-y1)≧0.2, and the second interlayer comprises a second superlattice including a series of Al x2 Ga 1-x2 N/Al y2 Ga 1-y2 N alternate layers, (x2-y2)≧0.2, wherein the average of x1 and y1 is larger than that of x2 and y2.

TECHNICAL FIELD

The present application relates to a semiconductor power device,especially relates to a semiconductor power device with an interlayer inthe buffer layer.

DESCRIPTION OF BACKGROUND ART

Due to the demand of the semiconductor power device used for switch inhigh frequency, group III-V semiconductor materials, such as GaN,recently used for power device operated in high frequency developsrapidly. Because group III-V semiconductor materials are capable offorming a two-dimensional electron gas (2DEG) due to the piezoelectriceffect in the junction, group III-V semiconductor materials with 2DEGhas the advantages of outputting high electrical current concentration,low switching losing, and operating in high voltage with thecharacteristics of the high mobility of the electrons, high electronsconcentration of 2DEG, and the low electrical resistance of GaN. Thus,group III-V semiconductor materials are suitable for power device.

Common power device comprises Bipolar Junction Transistor (BJT) andField Effect Transistor (FET), wherein BJT turns on and off bycontrolling the bias voltage of the two pn-junctions thereof and hascertain ratio of the output current to the input current, which iscurrent gain. FET turns on and off by controlling the input signal tochange the electrical field thereof and therefore the characteristic ofthe tunnel. Both BJT and FET have the breakdown voltage and leakagecurrent issues when they adopt group III-V semiconductor materials toimprove switch speed and efficiency. Especially, due to the latticemismatch between the substrate and the following growth epitaxialmaterial, the epitaxial quality such as the dislocation concentration ofthe epitaxial layer, significantly influences the value of the breakdownvoltage and leakage current of BJT or FET.

SUMMARY OF THE DISCLOSURE

A semiconductor power device, comprising: a substrate; a firstsemiconductor layer with a first lattice constant formed on thesubstrate, wherein the first semiconductor layer comprises a first groupIII element; a first grading layer formed on the first semiconductorlayer and comprising a first portion; a second semiconductor layer witha second lattice constant formed on the first grading layer, wherein thesecond semiconductor layer comprises a second group III element; and afirst interlayer formed in the first grading layer and adjacent to thefirst portion of the first grading layer, wherein a composition of thefirst interlayer is different from that of the first portion, and thefirst grading layer comprises the first group III element and the secondgroup III element, and concentrations of both the first group IIIelement and the second group III element in the first grading layer aregradually changed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G show the manufacturing process in accordance with the firstembodiment of the present application;

FIG. 2 shows the cross-sectional view of the semiconductor power devicein accordance with the first embodiment of the present application;

FIG. 3 shows the cross-sectional view of the semiconductor power devicein accordance with the first embodiment of the present application;

FIG. 4 shows the schematic diagrams of the distribution of gradualchange in accordance with the first embodiment of the presentapplication.

FIG. 5 shows the cross-sectional view of the semiconductor power devicein accordance with another embodiment of the present application.

FIG. 6 shows the cross-sectional view of the semiconductor power devicein accordance with still another embodiment of the present application.

FIG. 7 shows the cross-sectional view of the semiconductor power devicein accordance with yet another embodiment of the present application.

FIG. 8 shows the cross-sectional view of the semiconductor power devicein accordance with a further embodiment of the present application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present application will be described indetail with reference to the accompanying drawings hereafter. Thefollowing embodiments are given by way of illustration to help thoseskilled in the art fully understand the spirit of the presentapplication. Hence, it should be noted that the present application isnot limited to the embodiments herein and can be realized by variousforms. Further, the drawings are not precise scale and components may beexaggerated in view of width, height, length, etc. Herein, the similaror identical reference numerals will denote the similar or identicalcomponents throughout the drawings.

First Embodiment

FIGS. 1A-1G show the manufacturing process in accordance with the firstembodiment of the present application. As FIGS. 1A-1E show, firstly asemiconductor layer 4 is formed on a substrate 2, and then a first part106 of a first grading layer 6 is formed on the semiconductor layer 4.After that, a first interlayer 8 is formed on the first part 106 of thefirst grading layer 6, and then a second part 107 of the first gradinglayer 6 is formed on the first interlayer 8. The first part 106 and thesecond part 107 of the first grading layer 6 have different latticeconstant, and the first interlayer 8 comprises amorphous material. Inthe embodiment, the first part 106 and the second part 107 of the firstgrading layer 6 comprise the same chemical elements. After that, as FIG.1F shows, a tunnel layer 10 is formed on the second part 107 of thefirst grading layer 6. In the embodiment, the tunnel layer 10 and thesemiconductor layer 4 comprise different materials. And then, referringto FIG. 1G, an electron supplying layer 12 is formed on the tunnel layer10. In the embodiment, the tunnel layer 10 and the electron supplyinglayer 12 are formed of the materials with different lattice constants.Due to the piezoelectric effect and polarization caused by the differentlattice constants, a two-dimensional electron gas (2DEG) is formed inthe tunnel layer 10 for increasing the efficiency of electrontransportation. In the embodiment, the tunnel layer 10, the firstgrading layer 6, and the electron supplying layer 12 comprise groupIII-V elements, but the composition of the group III-V elements of thefirst interlayer 8 is different from that of the tunnel layer 10, thefirst part 106, the second part 107, or the electron supplying layer 12.In other embodiment, the first interlayer 8 is devoid of group IIIelements or group III elements which the first part 106 and the secondpart 107 comprise. In the embodiment, the thicknesses of the first part106 and the second part 107 can be the same or different in accordancewith the process conditions.

The material of the substrate 2 can be Sapphire, GaN, AlN or Si. Inother embodiment, a portion of the substrate 2 can be removed to shortenthe leakage current path of the semiconductor power device for reducingthe leakage current when forming the semiconductor power device. Thesemiconductor layer 4 is formed on the substrate 2 as a buffer layer forreducing the lattice constant difference between the substrate 2 and thefollowing growth epitaxial stack. In the embodiment, the material of thesemiconductor layer 4 is AlN and formed on the surface of the substrate2 formed of GaN. The first grading layer 6 formed on the semiconductorlayer 4 comprises the first part 106, the second part 107, and the firstinterlayer 8 wherein the first layer 8 can be amorphous material, andthe first part 106 and the second part 107 can be semiconductormaterial. Specifically, the first layer 8 comprises SiN, which can beamorphous material, and the first part 106 and the second part 107 areformed of AlGaN, which can be semiconductor material. In other words,the first part 106 and the second part 107 comprising group III-Velements are separated by the first layer 8 which comprises thecomposition of the group III-V elements different from that of the firstpart 106 or the second part 107. In other embodiment, the first layer 8can cover a portion of the first part 106, which means that a portion ofthe first part 106 and the second part 107 contact directly without thefirst interlayer 8 therebetween. In that case, the first layer 8 is apatterned structure, which is periodic, semi-periodic or aperiodic,formed on the first part 106. The first part 106 and the second part 107comprise the same characteristic of gradual change, which means the Alconcentration of AlGaN of the first part 106 or the second part 107decreases along the direction away from the substrate 2 and the Gaconcentration of AlGaN of the first part 106 or the second part 107increases along the direction away from the substrate 2. In other words,the concentration of Ga of the second part 107 is higher than that ofthe first part 106, and the Al concentration of the first part 106 ishigher than that of the second part 107. From the energy bandgap pointof view, the first part 106 has higher energy bandgap than that of thesecond part 107, and the bandgap decreases along the direction away fromthe substrate 2. Specifically, in the embodiment, the chemical formulaof the AlGaN of the first part 106 can be Al_(x1)Ga_(y1)N, and that ofthe second part 107 can be Al_(x2)Ga_(y2)N, wherein 0<x2≦x1≦1and0<y1≦y2≦1. That is, the first part 106 and the second part 107 comprisedifferent composition ratio of AlGaN in different position. To be morespecific, the second part 107 comprises more Ga and less Al than thefirst part 106. In another embodiment, the compositions of AlGaN of thefirst part 106 and the second part 107 in the neighboring region can bethe same. In other words, Al_(x1)Ga_(y1)N of some portion of the firstpart 106 and Al₂Ga₂N of some portion of the second part 107 can have thecharacteristics of x2=x1 and y1=y2. Since the Al concentration decreasesalong the direction away from the substrate 2 and the concentration ofGa increases along the direction away from the substrate 2 in both thefirst part 106 and the second part 107, if they have regions having thesame compositions, such regions should be t closest to each other in thefirst part 106 and the second part 107. In other words, where the firstpart 106 and the second part 107 directly contact the first interlayer 8should have the same chemical composition. In the embodiment, the secondpart 107 has larger lattice constant than that of the first part 106,the lattice constants of the first part 106 and the second part 107 arebetween the lattice constants of the tunnel layer 10 and thesemiconductor layer 4, and the lattice constant of the first interlayer6 is between the lattice constant of the semiconductor layer 4 and thelattice constant of the tunnel layer 10. In the embodiment, the firstpart 106 and the second part 107 of the first grading layer 6 compriseAlGaN. In another embodiment, the first part 106 and the second part 107can also comprise AlInGaN.

As above mentioned, by forming the first interlayer 8 having thecomposition of group III-V materials different from those of adjacentsemiconductor layers , the defects occurring in the adjacent thesemiconductor layer, such as the first part 106, are barred by the firstinterlayer 8 from extending to the second part 107 so the defect densityof the epitaxial layers is decreased effectively, the quality of theepitaxial layers is improved, and the issues of the leakage current andbreakdown voltage of the power device made of the epitaxial layers arefurther improved. The stack of the tunnel layer 10 and the electronsupplying layer 12 are formed on the first grading layer 6. In theembodiment, the tunnel layer 10 is made of GaN, and the electronsupplying layer 12 is made of AIGaN. A portion of the tunnel layer 10 isdoped with carbon and another portion of the tunnel layer 10 is undopedwith carbon, of which the portion undoped with carbon is adjacent to theelectron supplying layer 12, whereas the portion doped with carbon isfar away from the electron supplying layer 12. The thickness of theportion of the tunnel layer 10 undoped with carbon is about 10˜1000 nm,and preferably can be between 50˜150 nm. The piezoelectric effect andpolarization caused by the different lattice constants of the tunnellayer 10 and the electron supplying layer form the two-dimensionalelectron gas (2DEG) in the portion of the tunnel layer 10 undpoed withcarbon. Since the tunnel layer 10 and the electron supplying layer 12are the combination of GaN and AIGaN, the heterostructure formed of GaNand AIGaN has higher electron mobility and high concentration ofelectron carriers, which make the device formed of thereof having highpower and high efficiency. And, the heterostructure of GaN/AIGaN notonly provides higher electron mobility and high concentration ofelectron carriers, but also the power device formed of thereof iscapable of working with high frequency in high voltage and hightemperature. In the embodiment, the defects from the difference oflattice constants between the semiconductor layer 4 and the tunnel layer10 can be reduced by setting the first grading layer 6 therebetween. Inother words, the difference of lattice constants between thesemiconductor layer 4 and the first part 106 is larger than that betweenthe second part 107 and tunnel layer 10. Therefore, comparing with thelarger lattice constant difference of the tunnel layer 10 directlyformed on the semiconductor layer 4, the existence of the first gradinglayer 6 is able to prevent two epitaxial layers from continuouslygrowing epitaxial layers having substantially different latticeconstants so the quality of epitaxial growth is improved. As theaforementioned, the defects occurring in the first part 106 during thegrowth thereof are barred by the first interlayer 8 of the first gradinglayer 6 from extending to the second part 107 for improving the qualityof the epitaxial stacking layers. In the embodiment, the tunnel layer 10and the electron supplying layer 12 are formed of undoped semiconductormaterials. In other embodiment, the tunnel layer 10 further comprises asecondary electron supplying layer (not shown in the figures) betweenthe tunnel layer 10 and the first grading layer 6, and the material ofthe secondary electron supplying layer can be GaN doped with carbon orcomprise same AIGaN as that of the electron supplying layer 12. Thedifference of lattice constants between the secondary electron supplyinglayer and the tunnel layer 10 can increase the effect of piezoelectricpolarization and spontaneous polarization for increasing theconcentration of the 2DEG in the tunnel layer 10.

Second Embodiment

As FIG. 2 shows, in another embodiment of the application, a secondgrading layer 7 can further be formed on the first grading layer 6, andthen the tunnel layer 10 and the electron supplying layer 12 are formedon the second grading layer 7. The second grading layer 7 comprises samegroup III-V elements as that of the first grading layer 6. The secondgrading layer 7 comprises a third part 108, a fourth part 109, and asecond interlayer 9 between the third part 108 and the fourth part 109,wherein the third part 108 and the fourth part 109 comprise group III-Vsemiconductor materials and the second interlayer 9 comprises amorphousmaterial. In the embodiment, the second interlayer 9 is formed of SiN tobar the defects occurred in the third part 108 from extending to thefourth part 109. In other words, the third part 108 and the fourth part109 comprising group III-V elements are separated by the secondinterlayer 9 devoid of the group III-V elements of the third part 108and the fourth part 109. In other embodiment, the second interlayer 9 isdevoid of group III elements or devoid of the group III elements of thethird part 108 or the fourth part 109. In other embodiment, the firstinterlayer 8 and the second interlayer 9 can comprise the same materialor different materials. In the embodiment, the third part 108 and thefourth part 109 comprise same group III-V semiconductor materials andhave the same characteristic of gradual change as that the first part106 and the second part 107. That is, in the third part 108 and thefourth part 109, the Al concentration decreases along the direction awayfrom the substrate 2 and the Ga concentration increases along thedirection away from the substrate 2. From the energy bandgap point ofview, the first part 106 has higher energy bandgap than that of thesecond part 107 and the third part 108 has higher energy bandgap thanthat of the fourth part 109, and the bandgap decreases along thedirection away from the substrate 2. Therefore, the first part 106 hasthe highest Al concentration and the fourth part 109 has the highest Gaconcentration among the first part 106, the second part 107, the thirdpart 108 and the fourth part 109, and the chemical formula of the fourparts can be Al_(a)Ga_(b)N, wherein 0<a≦1 and 0<b ≦1. Further, the orderof the Al concentration contained in the four parts is that the fourthpart 109<the third part 108<the second part 107<the first part 106, andthe order of the Ga concentration contained in the four parts is thatthe first part 106 <the second part 107<the third part 108<the fourthpart 109. In other words, the range of the Al concentration contained inthe second grading layer 7 is different from that in the first gradinglayer 6; similarly, the range of the Ga concentration contained in thesecond grading layer 7 is also different from that in the first gradinglayer 6.

In the embodiment, the second interlayer 9 of the second grading layer 7is formed of the same amorphous material, such as SiN, as that of thefirst interlayer 8 of the first grading layer 6. In other embodiment,other grading layer can be further formed on the second grading layer 7,wherein the grading layer has or do not have an interlayer. Afterforming multiple grading layers, the tunnel layer 10 and the electronsupplying layer 12 are formed on thereof. In the embodiment, the thirdpart 108 and the fourth part 109 of the second grading layer 7 havelarger lattice constant than that of the first part 106 and the secondpart 107 of the first grading layer 6. And, the lattice constants of thefirst part 106, the second part 107, the third part 108 and the fourthpart 109 are between the lattice constants of the tunnel layer 10 andthe semiconductor layer 4. In the embodiment, the first grading layer 6and the second grading layer 7 comprise AIGaN. In other embodiment, thefirst grading layer 6 and the second grading layer 7 can also compriseAlInGaN.

In abovementioned embodiment, the semiconductor layer 4 formed of AlN isformed by importing trimethylaluminum and ammonia into a reactionchamber. Namely, the flow rate of trimethylaluminum is about 220 sccmand the flow rate of ammonia is about 1000 sccm for forming thesemiconductor layer 4 formed of AlN with a thickness of 150 nm. Thematerials of the first grading layer 6 and the second grading layer 7comprise group III-V semiconductor materials, such as AlGaN, wherein theAl concentration can be between 20% and 80%. In the embodiment, thefirst grading layer 6 and the second grading layer 7 are the gradinglayers formed of AIGaN which is formed by importing trimethylaluminum,trimethylgallium and ammonia into a reaction chamber. Furthermore, thecompositions of these grading layers can be gradually changed byadjusting the ratio of these gases. Specifically, the first gradinglayer 6 and the second grading layer 7 with different compositions areformed by importing trimethylaluminum, ammonia and trimethylgallium, ofwhich the ratio is about between 30:1:1000 and 65:25:4000, wherein theflow rate of trimethylaluminum is about 65˜300 sccm, the flow rate oftrimethylgallium is about 10˜25 sccm, and the flow rate of ammonia isabout 1000˜4000 sccm. In the embodiment, the tunnel layer 10 is formedof GaN, which is grown by metal organic vapor deposition with ammoniaand trimethylgallium, wherein the ratio of trimethylgallium to ammoniato be imported into the chamber is about 60:1, the flow rate oftrimethylgallium is about 130 sccm and the flow rate of ammonia is about6000 sccm, and the thickness of the tunnel layer 10 is between 25 nm and3000 nm. A portion of the tunnel layer 10 is doped with carbon andanother portion of the tunnel layer 10 is undoped with carbon, whereinthe portion undoped with carbon is far away from the second gradinglayer 7 and the portion doped with carbon is adjacent to the secondgrading layer 7. The thickness of the portion of the tunnel layer 10undoped with carbon is about 10˜1000 nm, and preferably can be between50˜150 nm. In the embodiment, the flow rate of the gases imported intothe reaction chamber and the ratio of the gases can be adjusted based onthe requirement.

As FIG. 3 shows, after forming the first grading layer 6, tunnel layer10 and the electron supplying layer 12 in the abovementioned embodiment,an electrode D, an electrode S and an electrode G are respectivelyformed on the electron supplying layer 12 as terminals for electricallyconnecting outside device and controlling the operating mode of thepower device and the electron distribution in the tunnel layer 10,wherein the materials of the electrode S, the electrode D and theelectrode G can be the alloy of Ni and Au. In one embodiment, theelectrode S and the electrode D form ohmically contact with the electronsupplying layer 12, and the electrode G forms Schottky contact with theelectron supplying layer 12. As FIG. 3 shows, after forming theabovementioned electrodes, a protective layer 14 is formed on thereofand covers the surface of the electron supplying layer 12 for preventingthe surface of the electron supplying layer 12 from reacting with theimported material in the process to influence the electricalcharacteristic of the tunnel layer 10 and make the electricalcharacteristic unstable. Namely, the protective layer 14 is used forprotecting the surface of the electron supplying layer 12 from outsideinfluence that affect the composition thereof. In the embodiment, thematerial of the protective layer 14 can be oxide, such as SiO_(x) orAlO_(x), or nitride, such as SiN_(x) or GaN_(x). Next, the protectivelayer 14 is etched to reveal a portion of the electrode S, the electrodeD and the electrode G for electrically connecting to outside. Theprotective layer 14 can cover most of the surfaces of the electrode S,the electrode D and the electrode G, and can also reveal the surfacesand the side surfaces, which is not coplanar to the surfaces, for makingthe outside electrical connection easier.

In the abovementioned embodiment, the characteristic of gradual changeof AIGaN or AlGaInN of the first grading layer 6 and the second gradinglayer 7 can be continuous type or discontinuous type, such as steppedtype. Referring to the gradual change types A to I shown in FIG. 4, thevertical direction shows the concentration of an element in the firstgrading layer 6 and the second grading layer 7. In the embodiment, thevertical direction represents the Ga concentration. The horizontaldirection shows the distance away from the semiconductor layer 4 or thesubstrate 2 in the first grading layer 6 and the second grading layer 7.Therefore, all the gradual change types show that the higher Gaconcentration the portion in the grading layer has, the farther awayfrom the semiconductor layer 4 the portion in the grading layer is. Ingradual change types A to D, the distribution of the Ga concentration iscontinuous gradual change, wherein the gradual change graph is linear orcurved. In gradual change types E and F, the distributions of the Gaconcentration are stepped type, wherein the difference between the twomodes lies in the amount of change of the Ga concentration per unitdistance, and the Ga concentration is constant in some region of thefirst grading layer 6 and the second grading layer 7. In gradual changetype G, the distribution of the Ga concentration is constant in oneregion and is changed in a curve form before or after the region. Ingradual change type H, the Ga concentration begins increasing linearlyand keeps constant after reaching a certain level. Thus, the Gaconcentration does not increase after the region. In gradual change typeI, the distribution of the Ga concentration is gradually changed in ajag form, namely, following a region where the Ga concentrationincreases to a certain level, the Ga concentration is decreased in theadjacent region, and the aforementioned pattern is repeated severaltimes. Furthermore, in the jag form, the region where the Gaconcentration is increased is larger than the region where the Gaconcentration is decreased. Namely, in the first grading layer 6 and thesecond grading layer 7, as the portion is away from the semiconductorlayer 4, the region where the Ga concentration is increased is largerthan the region where the Ga concentration is decreased, thus thefurther the portion is away from the semiconductor layer 4, the higherGa concentration the portion has. In these gradual change types, it isnot necessary that the minimum value of distribution of theconcentration is equal to zero. For example, in gradual change types Band E, in the portion of which the distance away from the semiconductorlayer 4 is zero, the concentration is not equal to zero. And, in thegradual change types, it is not necessary that the gradual change beginsfrom the portion of which the distance away from the semiconductor layer4 is zero. For example, in the gradual change type F, the concentrationbegins to change from the portion away from the semiconductor layer 4for a distance. In the other embodiment, the gradual change types shownin FIG. 4 does not limit the description of the gradual change types ofthe Ga concentration respectively in the first grading layer 6 and thegrading layer 7. For example, the distribution of the Ga concentrationin the first grading layer 6 can be the gradual change type A, and thedistribution of the Ga concentration in the second grading layer 7 canbe the gradual change type H; or the distributions of the Gaconcentration in both of the first grading layer 6 and in the secondgrading layer 7 are the gradual change type B. Namely, the distributionsof the Ga concentration in the first grading layer 6 and in the secondgrading layer 7 comprise the same or different gradual change types. Inother embodiment, the gradual change types shown in FIG. 4 can be thedistribution of the Ga concentration from the first grading layer 6 tothe second grading layer 7. For example, the distribution of the Gaconcentration can be the gradual change type H.

Third Embodiment

As FIG. 5 shows, the difference between the third embodiment and theabovementioned first embodiment is that the first interlayer 8 comprisesa superlattice including a series of alternate Al_(x)Ga_(1-x)N layer801/Al_(y)Ga_(1-y)N layer 802, and |x-y |≧0.2, wherein the thickness ofAl_(x1)Ga_(1-x1)N layer 801/Al_(y1)Ga_(1-y1)N layer 802 is between 1 nmand 100 nm, the Al_(x)Ga_(1-x)N layer 801 is closer to the semiconductorlayer 4 than the Al_(y)Ga_(1-y)N layer 802, and the series of alternateAl_(x)Ga_(1-x)N layer 801/Al_(y)Ga_(1-y)N layer 802 starts fromAl_(x)Ga_(1-x)N layer 801 adjacent to the first part 106. The first part106 and the second part 107 of the first grading layer 6 respectivelyhave an first interface 1061 and a second interface 1071 adjacent to thefirst interlayer 8, wherein the portion of first part 106 near the firstinterface 1061 is formed of Al_(a)Ga_(1-a)N, and the portion of thesecond part 107 near the second interface 1071 is formed ofAl_(b)Ga_(1-b)N, wherein the relationship of a and b can be a =b or a≠b. In another embodiment, the relationship of x, y, a and b can be x >a=b >y, a >x >y >b, or x >a >b >y.

FIG. 6 shows another embodiment, wherein the first interlayer 8comprises a series of alternate Al_(x)Ga_(1-x),N layer801/Al_(y)Ga_(1-y)N layer 802/Al_(z)Ga_(1-z)N layer 803, and |x-y |≧0.2,|y-z |0.2, wherein the Al_(x)Ga_(1-x)N layer 801 is the closest to thesemiconductor layer 4 and the Al_(z)Ga_(1-z)N layer 803 is the farthestaway from the semiconductor layer 4 among the Al_(x)Ga_(1-x)N layer801/Al_(y)Ga_(1-y)N layer 802/Al_(z)Ga_(1-z)N layer 803, and the seriesof alternate Al_(x)Ga_(1-x)N layer 801/Al_(y)Ga_(1-y)N layer802/Al_(z)Ga_(1-z)N layer 803 starts from Al_(x)Ga_(1-x)N layer 801adjacent to the first part 106. In another embodiment, the relationshipof x, y, z, a and b can be x>a=b≧y>z, x>y≧a=b>z, a>x>y>z>b, x>a>b≧y>z,x>a≧y>b >z, or x>y ≧a>b>z.

Fourth Embodiment

As FIG. 7 shows, the difference between the fourth embodiment and theabovementioned second embodiment is that both of the first interlayer 8and the second interlayer 9 comprise superlattices respectively. Thefirst interlayer 8 includes a series of alternating Al_(x1)Ga_(1-x1)Nlayer 801/Al_(y1)Ga_(1-y1)N layer 802, |x1-y1|≧0.2, wherein theAl_(x1)Ga_(1-x1)N layer 801 is closer to the semiconductor layer 4 thanthe Al_(y1)Ga_(1-y1)N layer 802, and the series of alternateAl_(x1)Ga_(1-x1)N layer 801/Al_(y1)Ga_(1-y1)N layer 802 starts fromAl_(x1)Ga_(1-x1)N layer 801 adjacent to the first part 106. The secondinterlayer 9 includes a series of alternating Al_(x2)Ga_(1-x2)N layer901/Al_(y2)Ga_(1-y2)N layer 902, |x2-y2|≧0.2, wherein theAl_(x2)Ga_(1-x2)N layer 901 is closer to the semiconductor layer 4 thanthe Al_(y2)Ga_(1-y2)N layer 902, and the series of alternateAl_(x2)Ga_(1-x2)N layer 901/Al_(y2)Ga_(1-y2) layer 902 starts fromAl_(x2)Ga_(1-x2)N layer 901 adjacent to the third part 108. In anotherembodiment, the average of x1 and y1 can be larger than that of x2 andy2.

The first part 106 and the second part 107 of the first grading layer 6respectively have a first interface 1061 and a second interface 1071adjacent to the first interlayer 8, wherein the portion of first part106 near the first interface 1061 is formed of Al_(a)Ga_(1-a)N, and theportion of the second part 107 near the second interface 1071 is formedof Al_(b)Ga_(1-b)N, wherein the relationship of a and b can be a=b ora≠b. In other embodiment, the relationship of x1, y1, a and b can bex1>a=b>y1, a>x1>y1>b, or x1>a>b>y1.

The third part 108 and the fourth part 109 of the second grading layer 7respectively have an third interface 1081 and a fourth interface 1091adjacent to the second interlayer 9, wherein the portion of the thirdpart 108 near the third interface 1081 is formed of Al_(c)Ga_(1-c)N, andthe portion of the fourth part 109 near the fourth interface 1091 isformed of Al_(d)Ga_(1-d)N, wherein the relationship of c and d can bec=d or c≠d. In other embodiment, the relationship of x2, y2, c and d canbe x2>c=d>y2, c>x2>y2>d, or x2>c>d>y2.

FIG. 8 shows another embodiment, the first interlayer 8 comprises afirst series of alternating Al_(x1)Ga_(1-x1)N layer801/Al_(y1)Ga_(1-y1)N layer 802/Al_(z1)Ga_(1-z1)N layer 803,|x1-y1|≧0.2, |y1-z1|0.2, wherein the Al_(x1)Ga_(1-x1)N layer 801 is theclosest to the semiconductor layer 4 and the Al_(z1)Ga_(1-z1)N layer 803is the farthest away from the semiconductor layer 4 among theAl_(x1)Ga_(1-x1)N layer 801/Al_(y1)Ga_(1-y1)N layer802/Al_(z1)Ga_(1-z1)N layer 803, and the series of alternateAl_(x1)Ga_(1-x1)N layer 801/Al_(y1)Ga_(1-y1)N layer802/Al_(z1)Ga_(1-z1)N layer 803 starts from Al_(x1)Ga_(1-x1)N layer 801adjacent to the first part 106. The second interlayer 9 comprises asecond series of alternate Al_(x2)Ga_(1-x2)N layer 901/Al_(y2)Ga_(1-y2)Nlayer 902/Al_(z2)Ga_(1-z2)N layer 903, |x2-y2|≧0.2, |y2-z2 |0.2, whereinthe Al_(x2)Ga_(1-x2)N layer 901 is the closest to the semiconductorlayer 4 and the Al_(z2)Ga_(1-z2)N layer 903 is the farthest away fromthe semiconductor layer 4 among the Al_(x2)Ga_(1-x2)N layer901/Al_(y2)Ga_(1-y2)N layer 902/Al_(z2)Ga_(1-z2)N layer 903, and theseries of alternate Al_(x2)Ga_(1-x2)N layer 901/Al_(y2)Ga_(1-y2)N layer902/Al_(z2)Ga_(1-z2)N layer 903 starts from Al_(x2)Ga_(1-x2)N layer 901adjacent to the third part 108. In one embodiment, the average of x1, y1and z1 is larger than that of x2, y2 and z2. In another embodiment, therelationship of x1, y1, z1, a and b can be x1>a =b ≧y1>z1, x1>y1≧a=b >z1, a>x1>y1>z1>b, x1>a >b≧y1>z1, x1>a >y1>b >z1, or x1>y1≧a >b >z1.The relationship of x2, y2, z2, c and d can be x2>c =d ≧y2>z2, x2>y2≧c=d >z2, c >x2>y2>z2>d, x2>c>d≧y2>z2, x2>c >y2>d >z2, or x2>y2≧c >d >z2.

As being understood by a person skilled in the art, the foregoingpreferred embodiments of the present application are illustrated of thepresent application rather than limiting of the present application. Itis intended to cover various modifications and similar arrangementsincluded within the spirit and scope of the appended claims, the scopeof which should be accorded the broadest interpretation so as toencompass all such modifications and similar structure.

What is claimed is:
 1. A semiconductor power device, comprising: asubstrate; a first semiconductor layer with a first lattice constantformed on the substrate; a first grading layer formed on the firstsemiconductor layer and comprising a first portion; a second gradinglayer formed on the first grading layer; a second semiconductor layerwith a second lattice constant formed on the second grading layer; afirst interlayer formed in the first grading layer and adjacent to thefirst portion of the first grading layer; and a second interlayer formedin the second grading layer; wherein the first interlayer comprises afirst superlattice including a series ofAl_(x1)Ga_(1-x1)N/Al_(y1)Ga_(1-y1)N alternate layers, (x1-y1)≧0.2, andthe second interlayer comprises a second superlattice including a seriesof Al_(x2)Ga_(1-x2)N/Al_(y2)Ga_(1-y2)N alternate layers, (x2-y2≧0.2,wherein the average of x1 and y1 is larger than that of x2 and y2. 2.The semiconductor power device according to claim 1, wherein thethickness of the Al_(x1)Ga_(1-x1)N/Al_(y1)Ga_(1-y1)N layer is between 1nm and 100 nm.
 3. The semiconductor power device according to claim 1,wherein the first grading layer further comprises a second portionopposite to the first portion adjacent to the first interlayer.
 4. Thesemiconductor power device according to claim 3, wherein the firstportion and the second portion comprise Al_(a)Ga_(1-a)N andAl_(b)Ga_(1-b)N respectively, and x>a=b>y.
 5. The semiconductor powerdevice according to claim 3, wherein the first portion and the secondportion comprise Al_(a)Ga_(1-a)N and Al_(b)Ga_(1-b)N respectively, anda>x>y>b, or x>a>b>y.
 6. The semiconductor power device according toclaim 1, wherein the first lattice constant is smaller than the secondlattice constant.
 7. The semiconductor power device according to claim1, wherein the first interlayer has a lattice constant, and the latticeconstant is between the first lattice constant and the second latticeconstant.
 8. The semiconductor power device according to claim 1,wherein the first grading layer and the second grading layer compriseAlGaN or AlGaInN, and the first interlayer and the second interlayercomprise an amorphous material.
 9. The semiconductor power deviceaccording to claim 8, wherein the amorphous material comprises SiN. 10.The semiconductor power device according to claim 1, wherein the firstsemiconductor layer comprises a first group III element, the secondsemiconductor layer comprises a second group III element, the firstgrading layer comprises the first group III element and the second groupIII element, and concentrations of both the first group III element andthe second group III element in the first grading layer are graduallychanged.
 11. The semiconductor power device according to claim 10,wherein the second grading layer comprises the first group III elementand the second group III element, wherein concentrations of both of thefirst group III element and the second group III element in the secondgrading layer are gradually changed.
 12. The semiconductor power deviceaccording to claim 11, wherein a lattice constant of the second gradinglayer is larger than the lattice constant of the first grading layer.13. The semiconductor power device according to claim 1, wherein theAl_(x1)Ga_(1-x1)N layer is closer to the substrate than theAl_(y1)Ga_(1-y1)N layer.